The present invention relates to a dielectric ceramic used in the microwave frequency region and a multilayer microwave device employing the dielectric ceramic.
In response to recent development of communications utilizing electromagnetic wave in the microwave frequency region, for example, a mobile phone, a portable phone and satellite broadcasting, there is a keen demand for compact equipments. To this end, each of components constituting each equipment should be made compact. In these equipments, dielectric material is incorporated, as a dielectric resonator, in a filter element or an oscillator element. In case an identical resonance mode is employed, size of the dielectric resonator is inversely proportional to square root of a dielectric constant of the dielectric material. Therefore, material having high dielectric constant is required for producing a compact dielectric resonator. In addition, in order to put a dielectric resonator to practical use, the dielectric loss should be low in the microwave frequency region, namely, the Q value should be high. Furthermore, change of resonant frequency with temperature should be small.
A number of ceramics for dielectric resonators have been developed so far. For example, U.S. Pat. No. 4,330,631 discloses BaO-TiO.sub.2 -Sm.sub.2 O.sub.3 type ceramic as a ceramic having an especially high dielectric constant. This ceramic has a dielectic constant of about 80, a high Q value of about 3000 at 2 to 4 GHz and a small temperature coefficient of resonant frequency. Meanwhile, BaO-PbO-TiO.sub.2 -Nd.sub.2 O.sub.3 type ceramic is reported as a ceramic having a dielectric constant of not less than 90 in Journal of American Ceramic Society, Vol. 67 (1984), p. 278-281.
Meanwhile, if conductive material and dielectric ceramic are of multilayer construction, the dielectric resonator can be made compact and have high functions. Conductive material for multilayer devices is required to have high conductivity for use at high frequency and therefore, should be Cu, Au, Ag or their alloy. On the other hand, dielectric ceramic for multilayer devices is required to be co-fired with the conductive metal and thus, should be fired under conditions in which the conductive metal is neither molten nor oxidized. Namely, dielectric material for multilayer devices should be sintered densely at low temperatures of not more than 1050.degree. C. Furthermore, when Cu is used as the conductive metal, characteristics of dielectric material for multilayer devices should not deteriorate even in the case of firing at low partial pressure of oxygen.
However, known ceramics for microwave, including the above mentioned dielectric ceramic have a high firing temperature of about 1300.degree. C. and thus, cannot be co-fired with the conductive metal having high conductivity, thereby resulting in failure in production of multilayer devices. Bi-based material is known as a dielectric ceramic suitable for low-temperature sintering. Especially, Bi.sub.2 (ZnNb.sub.2)O.sub.6 -Bi.sub.3 (Ni.sub.2 Nb)O.sub.9 disclosed in U.S. Pat. No. 4,638,401 is a dielectric material for a multilayer capacitor and can be sintered at about 950.degree. C. Furthermore, this dielectric material has a high dielectric constant of 90 and excellent temperature characteristics of the dielectric constant. However, not to mention this dielectric material, no dielectric ceramic having high dielectric constant is known which can be used at high frequency of not less than 100 MHz and sintered at low temperatures.